The present invention relates to a document size detecting device, more particularly, relates to a document size detecting device for automatically detecting a size of a document set on a contact glass.
In recent years, there has been proposed an image forming device for a copying machine or the like having an automatic paper feeding function for detecting a document size and automatically feeding from a cassette a paper sheet of the size corresponding to a preset copy magnification factor, and an automatic magnification factor setting function for automatically computing the copy magnification factor from the detected document size and paper sheet size to perform a magnification changing operation of an optical system.
Among known document size detecting devices for use in such type of image forming device are:
(1) A device wherein a sensor bar having a sensor at its front end is revolved parallel to a surface of the contact glass sheet in operative association with the action of closing the document holder, whereby the size of the document is automatically detected on the basis of the time needed until the document is detected (see Japanese Utility Model Application Laid-Open No. 201558/1982);
(2) A device comprising a colored member installed at a predetermined position on the document holder, a light emitter for radiating light to the colored member, a light receiver for receiving the reflected light from the colored member, and drive means for retracting the light receiver to a position not interfering with the movement of the document before the copying operation is performed, wherein the size of the document is automatically detected on the basis of signals from the light receiver corresponding to the portions obstructed and non-obstructed by the document, respectively (see Japanese Patent Application Laid-Open No. 22424/1981); and
(3) A device including a pair of optical sensors attached to a movable frame at predetermined positions thereon which supports the optical section, wherein one of said optical sensors detects the end of a document while the other optical sensor reads a mark on an indicator plate, whereby the size of the document is automatically detected (see Japanese Patent Application Laid-Open No. 48759/1982).
However, the document size detecting device (1) poses a problem that when the speed changes at which the document holder is closed, the document detecting time changes, resulting in erroneously detecting the size of the document.
Further, the document size detecting device (2) requires a special driving mechanism for retracting the light receiver, thus complicating the arrangement of the device. Furthermore, a period of time for retracting the light receiver is needed from completing the detection of the document size till starting the exposure of the document, so that there is a problem that the time is prolonged which is needed after keying operation for starting exposure of the document till completion of exposure of the document.
Every time the above-mentioned document size detecting device (3) senses the document size, the movable frame supporting the optical unit must be reciprocated once. Therefore, there is a disadvantage that the time after key-input for starting exposure of the document till completion of exposure is prolonged.